英文:
Type cast custom types to base types
问题
如何将自定义类型转换为interface{}，然后再转换为基本类型（例如uint8）？
我不能像uint16(val.(Year))这样直接进行类型转换，因为我可能不知道所有的自定义类型，但我可以在运行时确定基本类型（uint8、uint32等）。
有许多基于数字的自定义类型（通常用作枚举类型）：
例如：
type Year uint16
type Day uint8
type Month uint8
等等...
问题是如何将interface{}类型转换为基本类型：
package main
import "fmt"
type Year uint16
// ....
//还有许多其他基于uint8的自定义类型
func AsUint16(val interface{}) uint16 {
return val.(uint16) //FAIL: 无法将val（类型为interface{}）转换为uint16类型：需要类型断言
}
func AsUint16_2(val interface{}) uint16 {
return uint16(val) //FAIL: 无法将val（类型为interface{}）转换为uint16类型：需要类型断言
}
func main() {
fmt.Println(AsUint16_2(Year(2015)))
}
http://play.golang.org/p/cyAnzQ90At
英文:
How can I convert custom type to interface{} and then to base type (ex. uint8)?
I can't use direct cast like uint16(val.(Year)) because I may not know all custom types, but I can determinate base types (uint8, uint32,...) in runtime
There are many custom types (usually used as enums) based on numeric:
ex:
type Year uint16
type Day uint8
type Month uint8
and so on...
The question is about type casting from interface{} to base types:
package main
import "fmt"
type Year uint16
// ....
//Many others custom types based on uint8
func AsUint16(val interface{}) uint16 {
return val.(uint16) //FAIL: cannot convert val (type interface {}) to type uint16: need type assertion
}
func AsUint16_2(val interface{}) uint16 {
return uint16(val) //FAIL: cannot convert val (type interface {}) to type uint16: need type assertion
}
func main() {
fmt.Println(AsUint16_2(Year(2015)))
}
答案1
得分: 16
你可以使用reflect包来实现这个：
package main
import "fmt"
import "reflect"
type Year uint16
func AsUint16(val interface{}) uint16 {
ref := reflect.ValueOf(val)
if ref.Kind() != reflect.Uint16 {
return 0
}
return uint16(ref.Uint())
}
func main() {
fmt.Println(AsUint16(Year(2015)))
}
根据你的情况，你可能想要返回(uint16, error)，而不是返回空值。
https://play.golang.org/p/sYm1jTCMIf
英文:
You can accomplish this by using the reflect package:
package main
import "fmt"
import "reflect"
type Year uint16
func AsUint16(val interface{}) uint16 {
ref := reflect.ValueOf(val)
if ref.Kind() != reflect.Uint16 {
return 0
}
return uint16(ref.Uint())
}
func main() {
fmt.Println(AsUint16(Year(2015)))
}
Depending on your situation, you may want to return (uint16, error), instead of returning the empty value.
答案2
得分: 9
你为什么在问题中包含了Year？你是希望将任意事物转换为年份，还是将年份转换为uint16？
如果我假设你指的是后一种情况，那么最好使用以下方法：
func (y Year) AsUint16() uint16 {
return uint16(y)
}
这个方法不需要进行任何类型断言或反射。
链接：https://play.golang.org/p/9wCQJe46PU
英文:
Why did you include Year in the question? Are you hoping to convert arbitrary things into Years, or convert Years into uint16s?
If I assume you meant the latter case, then it would be better to use a method
func (y Year) AsUint16() uint16 {
return uint16(y)
}
which doesn't need any type assertions or reflection.
答案3
得分: 0
Go 1.18及以上版本
在引入泛型之后，现在更容易编写适用于任何定义类型的函数。
func AsUint16[T ~uint16](v T) uint16 {
return uint16(v)
}
func AsUint8[T ~uint8](v T) uint8 {
return uint8(v)
}
与interface{}不同，这种转换是有效的，因为类型参数明确限制为具有特定底层类型的类型，使用波浪符号~（更多信息：https://stackoverflow.com/questions/70888240/whats-the-meaning-of-the-new-tilde-token-in-go）。
缺点是您必须为每个底层类型编写不同的函数，如上面的uint16，uint8等。使用类型切换在函数体中可以使用联合约束，但在我看来，使用专门的函数更清晰 —— 结果是相同的。
英文:
Go 1.18 and above
After the introduction of generics, it's now easier to write such a function that works for any defined type.
func AsUint16[T ~uint16](v T) uint16 {
return uint16(v)
}
func AsUint8[T ~uint8](v T) uint8 {
return uint8(v)
}
Unlike with interface{}, the conversion works because the type parameter is explicitly constrained to types with that specific underlying type using tilde ~ (more info: https://stackoverflow.com/questions/70888240/whats-the-meaning-of-the-new-tilde-token-in-go).
The downside is that you must write a different function for each underlying type, as shown above with uint16, uint8, etc. A union constraint would work with a type switch in the function body, but in my opinion it's much clearer to use specialized functions — the result is the same.
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